Re: Performance effect of number of columns on SELECT clause?

I'd be ready to bet that the "Column1" he refers to is actually part of the index, and the table is not read at all, but just the index is read...

JP

Connor McDonald <connor_mcdonald_at_yahoo.com> wrote in message news:<3C7D59C5.1892_at_yahoo.com>...
> Pasi Kovanen wrote:
> >
> > We did some tests using different SELECT clauses and were
> > surprised to found how much the number of columns that
> > were selected affect the performance.
> >
> > For example, execution time of
> > SELECT column1 from foobar
> > seems to be almost constant whatever the size of data returned in
> > column1 is 20 - 400 bytes (only 5% decrease w/ 400 bytes).
> >
> > Wheres
> > SELECT column1, column2, ... column13 from foobar can take
> > even 40% longer though mo
>
> Is perhaps column1 indexed ? Try it with
>
> SQL> set autotrace on
>
> and see if you get the same explain plan for each.
>
> hth
> connor
Received on Thu Feb 28 2002 - 14:54:32 CST